This application proposes to examine the role that three glial cell populations play in the development of the corpus callosum. One of these populations form the glial sling, a group of cells that underlie the corpus callosum forming a "bridge" between the two hemispheres. Two newly discovered populations are the glial wedge, a group of cells near the midline that are proposed to deflect cortical axons medially and the indusium griseum, a group of glial cells located dorsal to the callosum. The three specific aims proposed in this new application are as follows: (1) characterize the role Emx1 plays in regulating the expression of guidance molecules derived from the glial sling; (2) determine whether two other midline glial populations are also important for midline guidance of cortical axons; and (3) examine the development of midline glial cells in mouse mutants exhibiting an acallosal phenotype i.e. the netrin-1 and DCC mutant mice.